PSN-L Email List Message
Subject: RE: New Lehman on line (almost)
From: steve hammond shammon1@.............
Date: Tue, 1 Oct 2002 14:44:10 -0700
Let me say up front, Jan will need to think about the critical damping at
some point and he has been given some excellent information by Chris and
Allen and I agree, Bob Barn's info is really worth the reading. However,
having built a few Lehman systems myself, and having built a few that
didn't work so well... I took Jan's coarsely tuned comment to mean that he
may still not be certain if the system is even working. Jan, if that is the
case this may help you get started:
Pull the boom back a few inches and eyeball it and see if it comes to rest
some where in the ballpark of 2 to 5 swings pass the center line. If it
does not, remove the damping magnet from the boom, set the top of the
vertical post 1/8-inch in the direction of the mass weight, level the boom,
swing the boom and check to see if the boom eventually returns to center.
Screw with it until it does. Then start adjusting the vertical post to the
rear (away from the mass weight) until the period is significantly longer
and the boom returns to center. As you adjust the post back, keep a level
on the boom itself and adjust it to be level. (Some PSN members like to
keep the boom at a very slight down angle. Don't ask why it's like fine
wine) Once you reach the 12-14 second period point each adjustment will
become much more critical. If you reach a point that the boom drops off
center to the left or right side of center and will not return to center,
you have overshot the zero point and adjusted the post into what could be
thought of as a negative zone and you need to move the top of the post back
towards the mass weight to recreate the pendulum effect. Once you achieve
the non-dampened adjustment, say a 10 - 30 seconds period, you can now
start adding the dampening. I consider a 18 - 30 second non-dampened period
to be the most desirable. If after setting the period you find that the
device goes to one of the stops over night, then reduce the period so that
the device is not so sensitive to changes in the local site such as rain
and water table changes. Now let's talk about initial damping settings.
Chris and Allen are right, the boom should never remain in oscillation. At
this point add damping until it comes to rest in 3.5 swings. Guy's, this is
the coarsely adjusted point I think Jan is initially trying to achieve.
Turn it on and let it rip. See what you record. Have some fun with it--
Record a few events and see what you get. In the mean time, take a look at
Bob's info and start thinking about tuning the device and setting the
critical damping. Once you know it works you can start to achive the fine
Chris, to answer your question about the standard advice of 3.5 swings, in
the good old days, I know I know, before the Internet, the original PSN
members would meet at members homes on occasion and when the discussion
would turn to damping as it always did, the standard of 3.5 swings past
center seemed to be the point that most of us would record and not miss an
event. Nothing is worst then to have an over dampened seismograph that
didn't record an event. It didn't make the adjustment right or wrong, it is
what seemed to work best on our home brew designs.
Regards, Steve Hammond PSN San Jose
From: Jack Ivey [SMTP:ivey@...........
Sent: Tuesday, October 01, 2002 5:09 AM
Subject: RE: New Lehman on line (almost)
In defense of the underdamped proposition, a slight amount of underdamping
(that produces a second peak about, say, 10% the size of the first peak)
produce only a mild peak in the frequency response, and in fact will extend
low frequency response of the system slightly. The main advantage of this
arrangement is that it is very easy to tell exactly how much damping you
If you adjust for critical or overdamped, you can only guess, and lots of
will end up with a massively overdamped system, reducing their low
It is important to use realistic deflections when measuring damping, I have
that if you use large (1/8") deflections, you sometimes get very different
damping than you do with micrometer deflections. Bob Barns' calibrator
PSN site) is an excellent way to produce these small test signals.
From: ACole65464@....... [mailto:ACole65464@........
Sent: Monday, September 30, 2002 10:05 PM
Subject: Re: New Lehman on line (almost)
In a message dated 10/01/2002 12:14:06 AM !!!First Boot!!!,
In a message dated 30/09/02, shammon1@............. writes:
The standard rule is to pull the boom back a few inches and let it go. The
should loose 30% of its motion on each swing past center and come to rest
in 3 1/2 swings.
I am puzzled as to where this *standard rule* is supposed to come
from? But using it will give you a quite seriously underdamped system! A
critically damped system experiences no oscillation at all. This is
in the maths.
This is important if you apply post processing to the recorded signal
with the assumption that it was critically damped to start with. It will
also give problems with the amplitudes and frequencies calculated in FFT
displays and may 'smear' P and S wave recordings.
A procedure to get critical damping could involve deflecting the beam
a very small amount (microns) and recording the amplifier output. You
progressively increase the damping until the arm just returns to the
position without having crossed the zero line. If you increase the damping
further, the arm will simply take longer to get back to zero. If you use
huge deflections like a few inches, you are likely to encounter non linear
effects which do not apply to the tiny (hopefully!) signals that we
It is helpful if the recording displays just what the earth is doing.
It is really not helpful if the system adds an oscillating tail to every
In support of what Chris has stated, please go to:
http://www.seismo.com/msop/msop79/inst/inst4.html#aa250 Go to section 4.5
for a text description, and then click on figure 4.5.1a to see how
are supposed to be damped. About Critical is the response you should
I hope this helps a little, the diagrams may not make much sense at first
but it shows how professional instruments (electromagnetic, aka Lehman
designs) are adjusted.
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